This invention generally relates to the inhibition of scale formation and deposition in aqueous systems by adding thereto an effective threshold amount of a phosphonocarboxylic acid and a polymeric phosphinocarboxylic acid that contains features of both phosphonates and polyacrylates. In a preferred embodiment, the addition of the active materials to an aqueous system results in synergistic results with respect to obtaining unexpectedly high levels of scale inhibition. Specific applications contemplated herein are aqueous systems used in recirculating cooling towers, desalination, oil field applications particularly in secondary oil recovery operations, flash distillation, as well as in aqueous systems such as sugar solutions.
The materials disclosed herein as being suitable in inhibiting formation and deposition of scale and particulate matter, are particularly suitable for use in aqueous systems where calcium sulfate scaling is a major problem, as in desalination, specifically reverse osmosis and flash distillation. Other suitable applications include the petroleum industry, pulp digesters, phosphoric acid production, and sugar evaporators.
Scale-forming salts can be prevented from precipitating by complexing the cations with chelating or sequestering agents so that the solubility of the reaction products is not exceeded. Generally, this requires stoichiometric amounts of chelating or sequestering agent with respect to the scale-forming cation, which amounts are not always desirable or economical.
More than 25 years ago, it was discovered that certain inorganic polyphosphates would prevent such precipitation when added in amounts far less than the concentrations needed for sequestering or chelating. When a precipitation inhibitor is present in a potentially scale-forming system at a markedly lower concentration than that required for sequestering the scale-forming cation, it is said to be present in a "threshold" amount. Threshold inhibition describes the phenomenon whereby a substoichiometric amount of a scale inhibitor can stabilize a solution from precipitation which solution can contain hundreds or thousands of parts of scale-forming ions. Threshold inhibition generally takes place under conditions where a few, i.e, 1 to 10 ppm, of a polymeric inhibitor will stabilize in solution from about 100 to several thousand ppm of a scale-forming mineral.
As already discussed above, whereas threshold inhibition occurs at substoichiometric ratios of inhibitor to scale-forming cation, sequestration requires a stoichiometric ratio of sequestrant to scale-forming cation to maintain that cation in solution. Generally, sequestering takes place at a weight ratio of threshold active compound to scale-forming cation components of greater than about ten to one, depending on the anion components in the water. Threshold inhibition, however, generally takes place at a weight ratio of threshold active compound to scale forming cation components of less than about 0.5 to 1.0. For example, a calcium sulfate solution containing 1820 ppm of calcium ions and 4440 ppm of sulfate ions is thermodynamically unstable. Unless a scale inhibitor is added, precipitation in such a system will take place within about one-half hour. To control precipitation of calcium sulfate from the supersaturated solution, the following two approaches are available:
(a) to complex or sequester calcium ions with a complexing agent such as ethylenediamine tetraacetic acid (EDTA) or nitrilotriacetic acid (NTA). Amount of each required to completely complex calcium ions would be stoichiometric, i.e, 1:1 ratio of Ca:EDTA or about 13300 ppm of EDTA to sequester 1820 ppm of calcium;
(b) on a threshold basis, one would need a substoichiometric amount of about 2 ppm of a polyacrylate to completely inhibit precipitation of calcium sulfate.
Therefore, on the basis of the above discussion, the tremendous difference between sequestration and threshold inhibition reflects the obvious advantages of the latter over the former.
Canadian Pat. No. 1,081,604 to Nass et al describes scale control in recirculating cooling towers operating with zero blowdown which is achieved by maintaining a certain level of a scale inhibitor in the cooling tower water and softening and returning a sidestream to the cooling tower water. At middle of p. 9 of this patent, a list of preferred scale inhibiting materials is given which includes phosphono tri-carboxylic acids as well as amino phosphonic acids, diphosphonic acids, polyphosphoric acids, polyol phosphate esters, amino phosphonates, maleic anhydride copolymers, and acrylic polymers. Although this patent is not specific in stating the particular mineral material against which the noted scale inhibitors are effective, it appears that the objective is the control of calcium carbonate scaling. This conclusion is based, inter alia, on statements appearing at bottom of p. 8 and top of p. 9 of the patent.
Canadian Pat. No. 1,117,395 to Dubin et al, pertains to scale prevention in industrial cooling waters using a phosphonocarboxylic acid and a polymer of acrylic acid. This patent is expressly directed to a composition for inhibiting the deposition of scale and sludge on the heat transfer surfaces of cooling water systems. This patent discusses scaling problems in recirculating cooling tower water systems where calcium carbonate scaling is the primary problem. Based on the disclosure and the data presented, this patent shows effectiveness of the composition against calcium carbonate scaling and there is no reason to assume that such a composition would be effective in applications where calcium sulfate scaling is the dominant problem. Furthermore, there is evidence to support the conclusion that effectiveness against calcium carbonate scaling implies relative ineffectiveness against calcium sulfate scaling.
U.S. Pat. No. 4,046,707 to Smith et al discloses the use of the polymeric phosphinocarboxylic acids that contain features of both the phosphonates and the polyacrylates in aqueous systems for inhibiting precipitation of scale-forming salts particularly calcium sulfate, magnesium hydroxide, and calcium carbonate salts. This patent, in its entirety, is incorporated by reference as if fully set forth herein.
A related copending patent application entitled "Calcium Sulfate Inhibition" filed for inventor Zahid Amjad claims the method of calcium sulfate scale inhibition in certain aqueous systems by the addition to an aqueous system a polyacrylic acid or its salt and a phosphonocarboxylic acid.